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Guide To
Grease Interceptors Eliminating the Mystery
Design & Operation
Standards
PDI- G101
A112.14.3
A112.14.4
Sizing & Placement
Maintenance
New Technology
Large Outside Interceptors
Codes & Regulations
Waste Water Treatment
THE PLUMBING & DRAINAGE INSTITUTE
800 Turnpike Street ~ Suite 300
North Andover, MA. 01845
Phone: 1-800-589-8956
Web: www.PDIonline.org ~ E-Mail: [email protected]
© 1998 The Plumbing & Drainage Institute Revised 2010
Acknowledgments
The Plumbing & Drainage Institute wishes to thank its member companies named below,
For their support and assistance in producing this very much needed Guide To Grease Interceptors:
Josam Company
Jay R. Smith Mfg. Co.
Wade Division – Tyler Pipe
Watts Regulator Company
Zurn Industries, Inc.
We particularly wish to thank Jon Wehrenberg whose work entitled Understanding Grease
Separation and Recovery formed the basis for our Guide and who wrote the initial draft.
Thanks to George Flegel for his efforts in the areas of technical accuracy and proof-reading
which are much appreciated. Many thanks to Cathy Ackil who produced the outstanding
Illustrations, and to Jerry McDanal, who contributed much effort to revising and fine-tuning
the text. Finally, thanks to Billy Smith, Jim Hadley, and Al Becker for checking the finished work.
Plumbing & Drainage Institute
William C. Whitehead
June 15, 1998
GREASE INTERCEPTORS:
ELIMINATING THE MYSTERY
In the late 1800’s Nathanial Whiting of
California patented the passive gravity
separator, a device known as a grease trap.
That device remains relatively unchanged
today as a means of separating fats, oils and
greases from wastewater. Despite the fact that
the technology works well and is little changed
from its original design, grease is a problem in
almost every locale in the country.
When grease enters the waste stream it
creates a variety of problems and once fats,
oils and greases have entered the waste
stream they are rarely suitable for recycling
because of contamination from metals,
chemicals and pathogens. Further, disposal
must be in accordance with local and federal
requirements. The acronym “FOG” is utilized
in most contemporary references to fats, oils
and greases because with the increased use
of vegetable oils and fat substitutes, greases
now consist of more than various
configurations of fats.
The first problem is not one which affects
the waste water system; it is the loss of a
potentially valuable resource. When recycled
before being in a drain, FOG can be used in a
variety of products such as soaps and
cosmetics, fertilizer, lamp oil, animal feeds and
munitions. Aside from the loss of a valuable
commodity, when FOG enters the waste water
stream there is a large and unnecessary
economic loss due to additional problems.
Grease can block pipes, can form aggregates
which in turn can also cause blockages, and
grease encapsulated solids can increase the
time and cost of treating the wastes at waste
water treatment plants.
The problems are not limited to any specific
size wastewater collection system or treatment
facility. Private systems such as septic
systems will fail and require costly repair or
replacement just as will large systems which
might be found in cities such as Chicago,
Phoenix, New York or Miami. Grease has been
known to cause blockages due to occlusion in
pipes many feet in diameter, and in the city of
Chicago (as an example) millions of dollars
have already been spent replacing large
sewers whose internal diameters can now be
measured in inches due to solidified grease.
All of the problems are unnecessary
because separating grease from waste water
is easily accomplished. Grease interceptors
function using gravity and coalescence as a
means of separation. Greases, fats and oils are
about 89 or 90% of the weight of water. To
separate them from water an interceptor
provides a separation chamber which allows
FOG to rise to the surface. FOG free water then
exits from the separation chamber at the low
point farthest from the inlet end.
To simplify the influences which affect
separation one could say there are only three
major factors which must be considered. They
are the design of the interceptor, the
installation of the interceptor, and the
maintenance of the interceptor.
G u ide To Gre a se Int ercep tor s 1
INTERCEPTOR DESIGN
Because of the scope of problems relating
to FOG, there are a large number of product
designs and offerings. Due to this fact it would
seem difficult for one to choose an interceptor
which would function as designed. Fortunately
that is not the case. See Figures 1 and 2.
In the early 1940’s the United States
government through the Army Corps of
Engineers, the Quartermaster General, the
Surgeon General, and the Research Committee
of the Plumbing and Drainage Manufacturer’s
Association (now the Plumbing and Drainage
Institute), and others held a series of
conferences to develop a testing program to
establish a means of rating flows and capacities
for grease interceptors manufactured at that
time.
From the efforts of the involved parties, and
as a result of exhaustive laboratory testing by
the Iowa Institute of Hydraulic Research at the
State University, a standard now known as PDI-
G 101 was developed. Since the first issue of
the PDI standard in 1949 it has been widely
recognized, and it is included as the basic
testing and rating requirement of Military
Specification MIL-T-18361
As a result of the existence of PDI-G 101 a
product which is designed to that standard and
is certified as having met that standard can be
installed with the confidence that it will be an
efficient separator at flow rates up to and
including its rated flow and up to and including
its rated capacity for retained FOG.
The products which are certified to PDI-G
101 are Hydro Mechanical Grease Interceptors
up to 100 GPM which are typically installed at
the fixture or the point of use. Since a PDI
certified interceptor is small, relatively speaking,
i t accomplishes its separation efficiency by
means of specially engineered internal baffling
arrangements used in conjunction with an
external vented flow control device.
Using the principles of fluid mechanics, a
PDI certified interceptor takes advantage of air
entrained in the effluent by the vented flow
control device to accelerate separation. See
Figure 3.
Figure 1:-Typical Grease Interceptor
G u ide To Gre a se Int ercep tor s 2
Figure 2: Grease Interceptor serving trapped and vented sink on floor above - flow control air intake intersects vent
FOG laden waste water passes through a
vented flow control device on its way into a PDI
certified interceptor. The flow control device
has an internal orifice which limits the flow into
the interceptor to the interceptor’s rated
capacity. As the effluent passes through the
orifice, which is sized to compensate for the
amount of head in the waste water collection
system, air is introduced through the vent
(which is actually an air intake). The entrained
air remains with the effluent until it enters the
grease interceptor.
Upon entering the grease interceptor, the
effluent is directed through the separation
chamber of the interceptor by means of a
system of baffles. The baffles serve to
lengthen the flow path of the effluent to
increase the time of separation while providing
a non-turbulent environment for separation to
take place. The entrained air will separate from
the effluent quickly. As it does so, it
accomplishes two things; First, the escaping air
accelerates the separation of FOG as it rises
rapidly to the surface of the water in the
separation chamber. The rising air bubbles
literally pull the FOG globules to the top of the
water. Second, the air released then provides
a small amount of positive pressure above the
contents of the separation chamber to regulate
the internal running water level of the grease
interceptor.
Most manufacturers provide methods to
regulate internal air pressure to prevent the
contents of the separation chamber from being
forced downward thus reducing the
interceptor’s capacity and efficiency. See Figure
4. Furthermore, most codes contain language
requiring a means of preventing the
interceptor from becoming air bound.
Typically, that language will state: “Venting.
Interceptors and separators shall be so
designed that they will not become air bound
when airtight covers are used”.
For the specifier or purchaser of a grease
interceptor to be assured the product will
perform as intended it is only necessary to
verify the product has been certified to a
known standard such as PDI- G101.
No discussion of the design of grease
interceptors would be complete without
covering large Gravity Grease Interceptors
which are typically located outdoors.
Figure 3: External vented flow control device with air and water flow shown. Actual configuration may vary from design shown.
Figure 4: Internal air relief detail.
G u ide To Gre a se Int ercep tor s 3
In certain areas of the country existing codes will
allow or even require an interceptor whose sole
specification is size. Unfortunately, even today,
more than 100 years since Nathanial Whiting
patented the grease interceptor there exists no
grease retention performance standard for or
base of accumulated data on large capacity
interceptors. There is a high expectation that
large capacity interceptors will work, but there is
no consensus standard or test data to stipulate
or verify their grease removal performance.
See Figures 5a and 5b.
Since remotely located outdoor (Gravity)
interceptors must deal with conditions different
from point of use interceptors the design
requirements will vary. One must first define
the required retention time based upon the
maximum anticipated rate of flow. This varies
from city to city or region to region so it must
be left up to the appropriate administrative
authority to establish this requirement through
testing based upon installation conditions.
These requirements currently vary from simple
statements (in local codes) of minimum
capacity size (such as 750 gallons) to retention
times based upon flow rates (such as 30
minutes) to formulas which make assumptions
about the amount of water used per meal
served. The lack of uniformity in sizing
requirements for remotely installed interceptors
is indicative of the lack of consensus about
their performance.
Figure 5a: A large capacity, in-ground type grease interceptor, typically concrete.
G u ide To Gre a se Int ercep tor s 4
Figure 5b: A large capacity, in-door or in-ground type Grease Interceptor, typically steel.
A properly sized and designed grease
interceptor may not work or may work less
efficiently if it is installed incorrectly. As basic as
it seems, the interceptor must not be installed
backwards. This is mentioned since far too
many interceptors which are condemned for not
working have merely been installed backwards.
The problems relating to installation, however,
go beyond the obvious. Regardless of whether
the interceptor is a certified Hydro Mechanical
Interceptor or a large Gravity interceptor, one of
the most important installation practices to
follow must be to locate the interceptor as near
as possible to the source of the FOG laden
water. See Figures 6 and 7. As stated
previously, this is important because every foot
of piping between the source of FOG laden
waste water and the interceptor is unprotected
and is a potential maintenance problem.
A second reason for locating the interceptor
near the fixture: FOG separates best when the
effluent is relatively hot.
While the laws of physics dictate that FOG
separates from water at a slower rate as
temperatures increase, in these applications
the separation rates at room temperature and
at elevated temperatures (testing has been
done up to 200 degrees Fº) are so close that the
other benefits outweigh the slight improvement
in separation rate. For example, in waste water,
particularly the FOG laden waste water from
commercial kitchens, it is likely there will be
solids present. These solids and the FOG are
more likely to form a globule, the specific gravity
of which exceeds that of FOG alone. As the
effluent temperature rises however, the FOG will
be more likely to separate freely from those
solids. Keeping the FOG from coalescing on
the solids is important because the resultant
INSTALLATION
G u ide To Gre a se Int ercep tor s 5
Figure 7: Grease Interceptors serving trapped and vented sinks in a multi-story installation – flow control air intakes intersect vent.
Figure 6: Grease Interceptor serving sink - flow control air intake intersects vent.
material may sink, and ultimately be discharged
from the interceptor. If on the other hand, the
FOG is free to separate from the solids in the
waste water due to the higher temperatures,
which tend to make the FOG less viscous, the
FOG is more likely to be retained in the
interceptor.
FOG laden solids passing through the
interceptor create two problems. First, they tend
to form balls or aggregates (grease can become
very hard) posing a blockage problem in the
waste water collection system. Second, if these
materials do make it to the waste water
treatment plant without creating any blockages,
they can make waste water treatment much
more difficult since degradation of FOG
consumes oxygen necessary for the digestion
of the waste in the treatment plant and because
FOG decomposition is quite slow, it can pass
through the plant. This increases the effort
required to treat wastes and can cause
violations of the plant’s discharge permit.
Unfortunately many of the codes in
existence around the country fail to recognize
the benefits of hot water in the FOG laden waste
stream and require oversized Gravity
Interceptors to allow the waste water to cool.
PDI has done extensive testing on the affect of
hot water on separation and can support through
data the fact that hot water has little effect on
separation efficiency. The Environmental
Protection Agency, in their document EPA
625/1-80-012 (Design Manual: Onsite
Wastewater Treatment and Disposal Systems)
is specific in recommending the use of hot
water and proximity to the source to enhance
retention of FOG.
When discussing the location as a factor in
installations, it should also be pointed out that
in addition to proximity to the fixture, the
interceptor should be located so that
maintenance can be easily performed.
Although this recommendation also seems so
obvious as to not need discussion, some
interceptors have been installed under sinks
without clearance for removal of the cover.
Some interceptors have been placed in the floor
and tiled over; some have been located so that
they are literally hidden from view; and some
large outdoor interceptors have actually been
paved over. The placement should allow the
cover to be visible and easily removable for
cleaning, and clearances should be such that
the internal baffling can be serviced. With the
cover removed, all wetted surfaces should be
visible. This is necessary not only for access to
clean the interceptor, but also to have the
capability to easily inspect the interior for
potential problems such as damaged baffles
and blocked air relief bypasses.
The flow control fitting furnished with PDI
certified interceptors must be installed in the
waste line ahead of the interceptor. It should be
located beyond the last connection from the
fixture and as close as possible to the
underside of the lowest fixture to minimize the
effects of head pressure. When the wastes of
two or more sinks or fixtures are combined to
be served by one interceptor, a single flow
control fitting may be used. Any flow control
fitting installation not in conformance with these
recommendations requires manufacturer
consultation.
The air intake for the flow control may
terminate under the sink drain board as high as
possible above the flood level of the sink in
order to prevent overflow. It may also terminate
in a return bend at the same height outside the
building. When the fixture is individually trapped
and back vented, the air intake may intersect
the vent stack. All installation recommendations
are subject to the approval of the local plumbing
code authority. See Figure 8.
G u ide To Gre a se Int ercep tor s 6
One of the most controversial issues re-
lating to Installation is: what fixtures or sources
must be part of the FOG interceptor system?
All drain-borne FOG is a problem and if the
problem is going to be solved all sources of
FOG must pass through the grease interceptor.
There is little controversy about connecting pot
sinks. There is some controversy about
connecting dishwashers. There are some
questions relating to floor drains, but discharge
from food grinders (or garbage disposals) is
almost universally required to bypass the
grease interceptor or to have the pulverized
solids removed from the waste stream before it
enters the interceptor.
The food grinder (and the associated pre-rinse
station at the dishwasher) is one of the single
greatest sources of FOG. Yet despite that fact,
most codes forbid food grinder discharge from
passing through a grease interceptor.
Technologically there is no reason for the waste
stream to bypass the grease interceptor if the solids
have been removed. See Figure 9.
G u ide To Gre a se Int ercep tor s 7
Figure 9: Grease Interceptors with solids interceptor servicing dishwasher with pre-rinse station and food grinder - flow control air intake terminates above flood level.
Figure 8: Grease Interceptor serving two individually trapped and vented sinks – flow control air intakes intersects vent.
Malls, Arena’s and Food Courts
Malls and Arena’s with multiple food service
establishments or food courts have some
unique FOG collection problems. The preferred
solution to protect the public sewer system is to
have each FOG producing establishment with
its own separate connection to the public sewer
system. Each food establishment can then be
held responsible for its FOG discharge. There
are however situations where the discharge
from multiple food service establishments are
combined prior to the final connection to the
public sewer system. This type of drainage
system is commonly found in malls and arenas
and other complexes with food courts. Often the
complex is responsible for the final discharge to
the public sewer system so they are required to
have a grease interceptor to accept all these
combined drainage lines before the connection
to the sewer system. This central collection
point just prior to the connection to the public
sewer system is monitored and maintained by
the complex management company and
protects the public sewer system. There are two
problems associated with this type of system.
The first problem is that the drainage system in
the complex and to the distant grease
interceptor is susceptible to FOG blockages.
The second problem is that it is difficult to hold
each food establishment to proper FOG
handling practices. Poor FOG handling
practices in one establishment can result in
shutting down a complete food court and sewer
over flows in the building. All food service
establishments in the common drain lines
should be using “Best Management Practices”
for their FOG handling but accountability is
difficult if not impossible in this case.
The solution is individual Hydro Mechanical
grease interceptors at each food service
establishment. The maintenance of each of
these interceptors can be the responsibility of
each food service establishment or the facility
management company. The addition of
individual interceptors will insure accountability
and reinforce the need for “Best Management
Practices” at each food service establishment.
G u ide To Gre a se Int ercep tor s 8
MAINTENANCE
Even the best designed interceptors, properly
installed will fail if they are not maintained. The
precise requirements for maintenance are not
possible to define since conditions at each
installation vary. In terms of the typical code,
maintenance must be performed before the
grease in the waste water down stream from
the interceptor exceeds local limits.
While that is a simple statement to make, it
is impossible for the user of a grease
interceptor to determine when those limits
have been exceeded. The method for
determining when an interceptor’s rated
capacity has been reached is fairly simple if it
is a PDI certified interceptor. A PDI certified
interceptor has a rated retention capacity equal
to twice its flow rate expressed in pounds. For
example, a 35 GPM interceptor is rated to
retain at least 70 lbs. of grease. A user may
determine a cleaning schedule by measuring
how much grease has been trapped over a
period of time.
Grease will weigh about 7 pounds per
gallon, and if it is determined that a 35 GPM
interceptor accumulates about 5 gallons of
grease every 4 days it would be easily and
correctly assumed that the interceptor must be
cleaned no less than once a week. In fact, if the
user must comply with a code which limits
grease to 100 parts per million, cleaning would
be recommended every 2 or 3 days. When
cleaning is discussed, it should be understood
that cleaning an interceptor should always
include the removal of grease from the top of
the separation chamber as well as any solids
which have accumulated along the bottom.
See Figure 10.
The actual frequency of cleaning a certified
interceptor will vary depending upon a wide
variety of factors; the type of food served will
determine how much grease will enter the
interceptor. An interceptor used for cleaning
utensils or limited to serving trays in a
restaurant where no food is actually prepared
is going to accumulate a lot less grease than
one used in a full service restaurant where all
of the food preparation equipment and utensils
as well as dishes are washed. Another factor
affecting the cleaning cycle will be whether a
food grinder is discharged into the interceptor,
and whether the food specialty is high in FOG.
The allowable grease content in the waste
G u ide To Gre a se Int ercep tor s 9
Figure 10: Grease Interceptor cleaning and maintenance.
water will also determine the frequency of
cleaning. It should be noted that all PDI
certified interceptors will separate efficiently
enough to meet any grease limits (which may
range from 50 parts per million up to as much as
600 parts per million depending upon the
jurisdiction). They may require cleaning when as
little as 25% of their rated capacity has been
reached depending upon the limits established
by the administrative authority. This statement is
based on an analysis by PDI of accumulated
test data. That data was collected at full rated
flows, and does vary from product to product.
The cleaning cycle on large capacity
interceptors is less easily determined.
Anecdotal evidence gathered from a variety of
sources and communities indicates that their
size is often interpreted as meaning less
frequent cleaning is required, and to a degree
this may be true. From information gathered
from a variety of sources however, the
consensus appears to indicate the cleaning
frequency for large interceptors is in the range
of 2 to 4 weeks. This amount of time is the
maximum allowable for large interceptors to still
meet the discharge limits on FOG. Due to the
nature of the large interceptors, the user is not
likely to be the cleaner, and in some cases
may actually be prohibited from cleaning the
interceptor. Usually cleaning will be done by a
renderer, a septic tank service, or a company
which specializes in grease interceptor
cleaning. The annual cost of regular cleaning
is likely to average between $2, 800 and $4,
000 depending again upon the discharge limits
and the local market costs. (January, 1998
average cost)
Regardless of what the cleaning cycle is
determined to be, it has been shown by actual
field experience that one of the biggest
obstacles to regular maintenance has been the
odors usually associated with interceptors. The
easiest way to eliminate that problem is
frequent cleaning. If cleaning the grease
interceptor becomes a part of the daily routine
it usually will only require about 15 minutes and
there will be limited or no objectionable odors.
It has been determined that when food
grinders are part of the waste system, and a
properly sized solids interceptor, cleaned daily,
is located ahead of the grease interceptor, the
odors normally associated with the grease
interceptor are not present because the food
particles which decay and cause odors never
reach the interceptor.
Use of the solids interceptor improves the
grease quality to extent that the recovered
grease may be disposed of with the golden
fryer grease which is usually purchased by the
local renderer. Now instead of paying for
disposal, the restaurant may be compensated
for the grease, since it can be recycled into a
variety of products.
When regular maintenance is not
performed the obvious result is a grease
interceptor which becomes unable to separate
the FOG due to overloading, thus passing
these materials downstream. Unless i t is
equipped with an electronic, sensor controlled,
positive inlet closure valve to prevent such
overloading, no grease interceptor wi l l
otherwise automatically shut itself down to
prevent overload discharge. Apart from
violating codes or ruining the on-site
wastewater treatment system, sewer blockages
and the associated health risks are likely.
Some FOG generators would rather do almost
anything but clean a grease interceptor. FOG
generators have several options, some of
which are acceptable alternatives, and some
of which are possibly legal, but nevertheless
unacceptable. One alternative is to engage the services of a company which specializes in cleaning
G u ide To Gre a se Int ercep tor s 10
interceptors. This is not an inexpensive approach, and in the case of large interceptors is required. If the service is performed as often as necessary, it insures the interceptor will function as intended.
Another alternative is the use of an
interceptor that is considered to be a Grease
Recovery Device (or Grease Removal Device).
A GRD is a Hydro Mechanical Grease
Interceptor which has as an integral part of its
design a means by which grease is removed.
A GRD will be one of two basic types:
1. Timer controlled - See Figure 11.
2. Sensor controlled - See Figure 12.
Timer controlled devices typically utilize a
disk or belt which passes through the FOG
layer and a squeegee device to wipe the
accumulated FOG from the disk or belt into a
drain trough and into a FOG receptacle. Other
means of removing the FOG include a pump or
gravity flow activated by the timer. They are
usually regulated by a 24 hour timer which is set
upon installation. The timer will operate the
FOG removal system for a set time or times
each day.
Sensor controlled devices have the ability to
sense the presence of FOG. By detecting FOG
and initiating the removal process only when
necessary and as often as necessary, the GRD
can always keep the retained FOG below the
rated capacity of the device. The sensor
operated devices use valving and gravity or
pump assisted FOG removal.
As FOG problems continue to be a factor, in
many jurisdictions the use of a GRD, is being
mandated. It must be noted that while a GRD
eliminates the daily routine of grease
interceptor cleaning, these devices do require
periodic maintenance to remove trapped solid
debris, removal of scum and a check of system
operation.
The previous two examples of methods to
avoid routine maintenance are certainly good
and acceptable choices. Some others are not
and are to be avoided in conventional grease
interceptors. The first is the use of chemicals,
often touted as environmentally friendly
enzymes or emulsifiers. These materials may
even have names which imply their use is
environmentally acceptable. The second is the
use of “bacteria” or organisms designed to
digest wastes.
Figure 11: A timer controlled Grease Recovery Device (GRD)
Figure 12: Sensor controlled Grease Recovery Device (GRD)
G u ide To Gre a se Int ercep tor s 11
In the first category, the materials used
work by changing the structure of FOG from a
hydrophobic material that is unlikely to mix
freely with water (thus allowing separation to
easily occur) to a hydrophilic micelle which
mixes freely with water thus inhibiting or
preventing separation from occurring in the
interceptor. The use of these additives only
changes the structure of the FOG for a limited
period of time, and eventually the FOG will
revert back to its original form, usually
downstream in the public waste water col-
lection system. While this practice, in conven-
tional interceptors, works to pass the problems
on to somebody else, the methods jurisdictions
use today to detect FOG content in the effluent
are sophisticated enough to accurately identify
any violator of the sewer codes.
The second method, the use of bacteria (or
bio-remediation as it is called) works. The
concept of bio-remediation is sound: trap
greases and digest them in the interceptor to
convert the grease permanently into the by-
products of digestion. This is exactly what
happens in a sophisticated waste water
treatment plant. See Figure 13. Bio-
remediation does not eliminate the need for
monitoring the effluent quality, routine
maintenance to deal with undigested
materials, or inspections to insure all
components are clean and functioning
properly.
For an additive to have any positive effect, it
must be known to produce net reduction in
weight and volume of the FOG either through
biochemical or catalytic processes. Such
disposal methods require engineered devices
and professional administration.
When dealing with a conventional grease
interceptor, the most practical and economic
maintenance practice is to regularly remove the
FOG and dispose of i t in accordance with
applicable solid and special waste disposal
regulations.
Figure 13: Bio-remediation Grease Interceptor.
G u ide To Gre a se Int ercep tor s 12
S U M M A R Y
The problems relating to fats, oils and
greases (FOG) are easily addressed, to do so
requires an understanding of the principles of
separation and a willingness to do all that is
necessary. Dealing with FOG problems is not
limited to restaurant owners; it is an issue a
number of parties must share in resolving.
Codes must be written or, more precisely,
rewritten to be technically correct.
Administrative Authorities must make certain
when they write and/or endorse codes that all
of the issues have been correctly addressed.
Interceptors and FOG disposal systems
which have been properly designed and
certified must be required and used. They must
be installed as they were tested and were
intended to be installed. And last, but not
least, the devices must be maintained
according to the codes and the manufacturer’s
requirements.
REMEMBER: Proper maintenance of even the
poorest interceptor wi l l provide better results
than the lack of maintenance on the best
interceptor.
G u ide To Gre a se Int ercep tor s 13